In a continuous ink jet printer, a continuous jet of ink is expelled from an orifice in a print head to form an ink jet filament. The ink jet is stimulated by a periodic disturbance induced by a stimulation signal applied to the ink jet head, to cause the ink jet to reliably break up into an evenly spaced series of drops. The distance from the orifice to the point of drop separation is called the ink jet filament length. The trajectories of the ink drops are controlled by inducing charge in the conductive ink jet filament at the moment of drop separation by means of a charging electrode located near the point of drop separation. Charge induced in the ink jet filament is trapped in the ink drops as they separate from the filament, and the path of a drop is determined by the interaction of the charged drop with a local electric field.
In a "binary" type continuous ink jet printer, drops are either charged or not by applying a voltage pulse to the charging electrode. Charged drops are deflected into a catcher from which the ink is recirculated, and uncharged drops proceed to an ink receiving surface, such as paper. In another type of continuous ink jet printer, printing drops are deflected along a plurality of paths determined by the amount of charge induced on the drops, more highly charged drops being deflected more than less highly charged drops.
In such continuous ink jet printing, the length of the ink jet filament varies as a function of the amplitude of the stimulation signal applied to the ink jet print head. Variables such as ink pressure, temperature, and viscosity affect the relationship between the filament length and the stimulation amplitude. Generally, at low stimulation amplitude, the ink jet filament is relatively long. As the stimulation amplitude is increased, the filament shortens until a minimum filament length is reached. As the stimulation amplitude is further increased, the ink jet filament begins to lengthen again. It has been found through experimentation that in this region of operation beyond the minimum filament length (called overdrive), ink drop satellites are uncontrollably formed. This makes accurate ink drop control difficult. It is therefore desirable to avoid operating the ink jet head in the region of overdrive.
There is a need therefore to have a means for sensing the filament length as a function of stimulation amplitude so that the stimulation amplitude applied to the ink jet print head may be automatically adjusted so as to avoid the region of overdrive. This is especially true since the relationship between the filament length and stimulation amplitude changes due to changes in ink viscosity, temperature and pressure.
U.S. Pat. No. 4,417,256 issued Nov. 22, 1983 to Filmore et al discloses a system for adjusting the stimulation amplitude in a multi-jet continuous ink jet printer. In the apparatus disclosed by Filmore et al, the filament length is inferred from a measurement of the time of flight of an ink drop, the longest time of flight corresponding to the shortest filament length, and vice versa. The time of flight is determined by measuring the time it takes for a charged drop to reach a drop sensor located downstream from the charging electrode. The starting time t.sub.0 is taken as the midpoint of the charging signal.
Because the actual starting time of the ink drops (i.e., the time that the drops break off from the filament, called the "separation phase" of the drops) is a function of the stimulation amplitude, this method of inferring filament length actually measures a function of both filament length and drop separation phase as a function of stimulation amplitude, and is effective only for comparing one filament length to another, but not for measuring the length of any one ink jet filament. Furthermore, in the method of stimulation setting proposed by Filmore, some of the ink jets are overdriven, thereby risking the formation of satellites.
It is therefore the object of the present invention to provide a method and apparatus for measuring filament length in an ink jet print head as a function of stimulation amplitude independent of drop separation phase.
It is a further object of the invention to provide a method and apparatus for automatically adjusting stimulation amplitude as a function of filament length independent of drop separation phase in an ink jet print head.